In recent years, nonaqueous electrolyte secondary batteries using metallic lithium, an alloy capable of storing and releasing lithium or a carbon material as the negative active material and a lithium transition metal complex oxide represented by the chemical formula: LiMO2 (M indicates a transition metal) as the positive active material have been noted as high-energy-density batteries.
A representing example of the lithium transition metal complex oxide is lithium cobalt oxide (LiCoO2), which has been already put to practical use as the positive active material for nonaqueous electrolyte secondary batteries. However, in the case where lithium cobalt oxide is used alone, batteries show capacity reduction during charge-discharge cycles, due to structural degradation of lithium cobalt oxide or decomposition of an electrolyte solution on a surface of a positive electrode.
An attempt has been made to improve such capacity reduction by substituting a metal for a part of cobalt. In Patent Literature 1, substitution of tungsten, manganese, tantalum, titanium or niobium for a part of cobalt has been studied. A further attempt has been made which incorporates an element, other than cobalt, in a positive electrode. In Patent Literature 2, addition of zirconium to lithium cobalt oxide has been studied.
However, the nonaqueous electrolyte secondary battery disclosed in Patent Literature 1 shows deteriorated charge-discharge cycle characteristics when its end-of-charge voltage is prescribed at 4.3 V or above, which has been a problem.
For nonaqueous electrolyte secondary batteries using a lithium transition metal oxide, such as lithium cobalt oxide, as the positive active material and a graphite material or the like as the negative active material, an end-of-charge voltage is generally prescribed at 4.1-4.2 V. In this case, the active material of the positive electrode utilizes only 50-60% of its theoretical capacity. Accordingly, if the end-of-charge voltage is increased to a higher level, a capacity (utilization factor) of the positive electrode can be improved to increase the battery capacity and energy density. However, a deeper depth of charge of the positive electrode, as a result of the increase of the end-of-charge voltage of the battery, increases a tendency of an electrolyte solution to decompose on a surface of the positive electrode and renders the positive active material more prone to experience structural degradation. As a result, more significant deterioration occurs during charge-discharge cycles, compared to the conventional case where the end-of-charge voltage was prescribed at 4.1-4.2 V.
In the nonaqueous electrolyte secondary battery disclosed in Patent Literature 2, an attempt to improve its charge-discharge cycle characteristics has been made by heat treating a mixture of a lithium salt, cobalt carbonate (CoCO3) and a zirconium compound to cover a surface of lithium cobalt oxide as by a zirconium oxide (ZrO2) so that decomposition of an electrolyte solution on a surface of the positive electrode is retarded and degradation of crystal structure of the active material of the positive electrode is suppressed.
However, in the nonaqueous electrolyte secondary battery manufactured by the method disclosed in Patent Literature 2, a surface of lithium cobalt oxide is covered with a non-ion-conducting zirconium compound (ZrO2 or Li2ZrO3). This deteriorates charge-discharge characteristics of the positive active material itself and accordingly of the battery, which has been a problem.    Patent Literature 1: Japanese Patent Laying-Open No. He 3-201368    Patent Literature 2: Japanese Patent Registration No. 2855877